1. Field of the Invention
This invention relates to processes for adhering articles together. More particularly, such processes are disclosed wherein articles are fused together employing vitrifiable adhesive compositions. Processes wherein fixturing of articles is accomplished prior to vitrication are also disclosed.
2. Description of the Prior Art
It has long been desired to provide processes for the adhering together of objects employing resinous adhesives which are suited to exposure to high temperatures. Such high temperature stable adhesives, however, are rare. At present, only a limited number of adhesives such as zinc and other metal phosphates and certain epoxy and phenolic resins are well adapted for resistance to thermal degradation; the upper temperature operating limits for these compositions is relatively modest. Accordingly, when adhesives or bonding agents are desired which are stable to high operating temperatures, such as, for example, in excess of about 300.degree. C., use of sauerreisen cements and similar inorganic filling materials has been resorted to.
As is well known to those skilled in the art, the sauerreisens are aqueous based, inorganic materials which may be used to bond together articles. Accordingly, such sauerreisens are applied to the articles to be bonded together and the material exposed to high temperature, usually through firing. The sauerreisens are thus caused to fuse to afford bonding between or among the articles. The sauerreisens and similar cements may be used only according to processes which are difficult to practice and which result in bonded species which are lacking in certain desired physical or electrical characteristics. Thus, sauerreisen-bonded articles have significant water-sorptivity. In this regard, such processes provide bonds which tend to be partially conductive of electricity when employed under conditions of high humidity and are, accordingly, unsuitable for certain electrical applications. For example, the Federal Aeronautics Administration requires that cements used in the construction of lamp units for runway illumination at airports in the United States have a value of 5,000 Megohms when tested under the Megger insulation test procedure. See in this regard, Megger.RTM. Manual, James G. Biddle Co. Philadelphia, Pa. (1954). At present, no known adhesive composition, resinous or otherwise, passes the F.A.A. Megger test while retaining good physical properties at the elevated temperatures experienced by runway lamps. Those skilled in the art will readily appreciate that numerous other uses exist for processes which provide high temperature stable adhesives and bonding agents which have good electrical insulative values under conditions of high humidity.
U.S. Pat. No. 3,901,752 issued to Taylor discloses the employment of mixtures of pyrolizable polymeric binders and polymerizable organic compositions for the in situ preparation or adhesive compositions at high temperatures. Such mixtures are especially suitable for laminations.
U.S. Pat. No. 3,728,185 issued to Gray discloses compositions having inorganic particulates dispersed in a binder comprising an olefin-SO.sub.2 copolymer. Upon heating, SO.sub.2 and olefin are liberated and the inorganic material fused to effect sealing.